An ossicular replacement prosthesis (ORP) is a structure that replaces part or all of one or more of the three bones of the ossicular chain of the human middle ear. A schematic representation of the ossicular chain is shown in FIG. 1. The ossicular chain, generally represented by reference numeral 1, comprises three connected bones, called auditory ossicles, which extend across the middle ear from the tympanic membrane (eardrum) 3, to the oval window (not shown). The bones include the malleus 5, which has landmark portions known as the neck or handle (more strictly the manubrium) 7, and the head 9; the incus 11, which includes a body portion 13; and the stapes 15. These bones are also known colloquially and respectively as the hammer, anvil and stirrup. The malleus 5, engages the eardrum 3, and is articulated to the incus 11, via the incudomalleal joint (IMJ) 17. The incus 11, in turn, is articulated with the stapes 15, via the incudostapedial joint (ISJ) 19, and the footplate 21, of the stapes 15, engages the oval window (not shown). Sound induced vibration of the eardrum 3, is thus transmitted across the ossicular chain 1, to the cochlea (not shown) of the inner ear.
ORPs are used in ossicular chain reconstruction in cases where the normal process of sound conduction from the eardrum to the inner ear is impeded by a failure of part or all of the ossicular chain to transmit the vibrations generated from sound arriving at the eardrum. The ORP provides a sound conduction bridge across the gap created when the dysfunctional parts of the ossicular chain are removed.
An ORP is implanted using a surgical procedure in which the middle ear is usually approached via the external ear canal and an incision is made around the eardrum which is then reflected to provide access to the middle ear cavity.
ORPs are classified by some worker in this field as Total ORPs (TORPs) and Partial ORPs (PORPs), the former spanning the complete ossicular chain from the eardrum to the oval window and the latter spanning part of the ossicular chain. There are also acronyms for other types of PORP. For example a PORP that replaces an incus (an incus replacement prosthesis) may be referred to as an IRP. In addition, special prostheses for treating otosclerosis are used in stapedectomy surgery. However, the term ORP as used herein is intended to refer generally to any device which is used to replace part or all of the ossicular chain.
An ORP must possess a number of properties in order to function effectively. For example, those parts which are exposed to the internal environment of the body and those in direct contact with body tissue must be biocompatible and chemically stable. Additionally, an ORP should be rigid enough and have sufficiently low mechanical damping to transmit acoustically derived vibrations with minimal or low loss of signal. Furthermore, an ORP should not impart any more loading to the ossicular chain by virtue of its mass than is absolutely necessary or clinically acceptable. During its lifetime, which may be many years, an ORP may be subjected to billions of vibrating cycles in the audio frequency range and must, therefore, maintain its structure and function with minimal deterioration.
Existing ORPs are manufactured in a wide range of shapes and sizes, and are designed to span the gap left after the excision of a portion or all of the ossicular chain. ORPs have a wide range of end-fittings constituting attachment means for attachment to the bones at each side of the gap. There are various means used in ORPs for attachment to the ossicular chain, including, non-exhaustively, wires, spring clips and other components which may be crimped into place. These attachment means form part of the prosthesis and usually rely on a lock or a friction fit between the prosthesis and the tissue. Cements constitute generally less-preferred attachment means and very few of them are approved for use in the middle ear by regulatory bodies, such as the Food and Drug Administration in the USA. Bioactive materials, such as calcium hydroxyapatite (syn. hydroxylapatite), form a chemical bond between bone and the implant and may be used alone or in a hybrid form with another means of attachment.
Some ORPs are offered in a considerable range of sizes whereas others are presented in only a few sizes or in a single size which can be modified to fit the patient intraoperatively by reshaping or by size reduction (cutting down). However, intraoperative modification may be difficult and time consuming.
In addition to transforming sound into a suitable form of vibration to activate the cochlea in the inner ear, the middle ear also has means for compensating for large static, or quasistatic, variations in atmospheric pressure (alternatively herein termed ambient pressure) which could otherwise cause sufficiently large and potentially damaging displacements of the ossicular chain.
Examples of commonly experienced effects of ambient pressure variations include passing into and out of a railway tunnel whilst on a train running at high speed, travelling in an aircraft during ascent and descent, underwater diving and nose-blowing. Compensation for ambient pressure changes is achieved by various means including the ability of the IMJ (17, in FIG. 1) to move in such a manner that high levels of deflection of the eardrum passed to the malleus are not transmitted via the incus to the stapes.
Implantation of all TORPs and most PORPs involves removing the incus, thereby destroying the ISJ and IMJ, which, in turn, results in the loss of the motion-limiting compensation mechanism. Accordingly, an ORP which provides effective means for preventing large changes in atmospheric pressure leading to excessive motion at the stapes footplate (21 in FIG. 1) is desirable. For example, a spring element forming part of an ORP could deflect under a force generated by a static pressure and so provide such compensating means. Examples of ORPs that employ spring elements are disclosed in U.S. Pat. Nos. 6,203,571 B1, 4,957,507, 4,624,672, WO 92/18066 and FR 2691354. However, the use of a spring element is not now generally regarded in the art as an optimal solution. For example, it has been pointed out by Bornitz et al (Design Considerations For Length Variable Prostheses: Finite Element Model Simulation, Middle Ear Mechanics in Research and Otology, eds. K. Gyo and H. Wada, World Scientific Press, 2003) that other means of adjustment to the length of an ORP would be desirable. Their proposals include a spring element, a damping element, a friction element and a buckling element. They conclude that a combined spring and damping element would be the best option for this purpose, although they concede that no realisation of such a device is yet known.
It is among the objects of embodiments of the present invention to seek to address these and other limitations in the prior art.